This invention relates to a satellite communication system of a space division multiple access-time division multiple access (abbreviated to "SDMA-TDMA" hereinafter) type and, in particular, to a method, a satellite circuit arrangement, and a synchronization circuit of an earth station for use in the SDMA-TDMA communication system. The SDMA-TDMA communication system, per se, is known and described in U.S. Pat. No. 3,928,804 (Schmidt, et al), U.S. Pat. No. 4,181,886 (Cooperman), or the like, and that the synchronization circuit is also known and described in U.S. Pat. No. 4,004,098 (Shimasaki).
In a satellite communication system of the type described, a satellite comprises a plurality of spot beam antennae which cover individual terrestrial zones, respectively. Each terrestrial zone comprises at least one earth station. The earth station or stations in each zone are for sending an up-link sequence of time division multiple access (TDMA) signals to the satellite and receiving a down-link sequence of TDMA signals from the satellite. Each TDMA signal of each down-link sequence includes down-link frame alignment bursts at a predetermined frame period.
The satellite comprises a memory for memorizing a switching format and for producing a sequence of switching signals in accordance with the memorized switching format. A switching matrix on board the satellite distributes the TDMA signals of the up-link sequences to the down-link sequences in accordance with the control signal sequence.
Sometimes, it happens that the switching format must be changed or switched to another switching format. The former format will be called a first switching format and the latter, a second switching format. In preparation for the change of formats, a satellite circuit arrangement disclosed in U.S. Pat. No. 4,181,886 issued to Richard S. Cooperman and assigned to Communications Satellite Corporation, comprises an additional memory for memorizing the second format in addition to the memory for memorizing the first switching format.
The second switching format is sent from a specific one of all earth stations to the additional memory, as a format specifying signal. The first and the second switching formats are switched from one to the other in response to a format switching command which is sent from the specific earth station to the satellite. The switching format selected by the command will be called an "on-line or active switching format" and the other, "a stand-by switching format."
Each of the transmitting and receiving sections of each earth station is put into operation in accordance with a station control format which is matched to the on-line switching format and which is peculiar to each earth station. During the time that the first switching format is used on board the satellite as the on-line switching format, the station control format should comply with the first switching format. As soon as the on-line switching format is switched to the second format, the station control format should be changed in each of the transmitting and the receiving sections to another station control format which is matched to the second switching format. Therefore, each earth station has to monitor the change on the satellite from one of the first and the second switching formats to the other. Otherwise, communication is objectionably interrupted.
A satellite is far from the earth surface. For example, a geostationary satellite is located at a distance of about 35,000 kilometers above the equator of the earth. A time division multiple access signal sent from an earth station returns to the earth station, under consideration, after a considerably long delay. The delay is usually longer than the frame period. The delay varies from a terrestrial zone to another and fluctuates dependent on movement of the satellite.
It is assumed that the switching and control formats are changed when a format switching command is received from a specific one of the earth stations to the satellite and the transmitting and receiving sections of each earth station. Inasmuch as the format switching command reaches the satellite and the earth stations with different timing, each of the switching and the control formats is individually and arbitrarily switched from one to another. It is difficult to concurrently change all of the switching formats and the control formats without any interruption of communication.